The 400V will be across the electronic load, regardless of how much current the load is drawing, within the specs of the power supply of course.

While you can certainly use a (400V - 30V) / 0.4A = 925 Ohm resistor to make sure the the transistor does not see more than 30V at max rating, what happens if the transistor for some reason conducts a bit less (due to noise or temperature change or whatever)? Let's say it drops down to conduct only 0.38A. Then you got 0.38A * 925 Ohm = 351.5V across the resistor, which means the remaining 400V - 351.5V = 48.5V is across the transistor... woops!

While you can certainly use a (400V - 30V) / 0.4A = 925 Ohm resistor to make sure the the transistor does not see more than 30V at max rating

Doh, late at night

Still the point stands. At max rating of 0.4A, let the transistor conduct fully, so it has effectively 0V across it. Then the resistor must be 400V / 0.4A = 1k Ohm. As you reduce the current through the load, the voltage across the transistor will increase. The transistor can handle max 30V, at which point 370V will be across the resistor. The current at that point is 370V / 1k Ohm = 0.37A. If you reduce the current through the load further, you'll blow your transistor...

The transistor and resistor form a potential divider. When the transistor is fully on, then it has very little voltage across it and all of the 400V is across the resistor, but when the transistor is off, it will have 400V across it.

You won't be able to find a BJT rated to take 400V and 0.4A simultaneously, the safe operating area of any BJT will be exceeded, even 200V at 0.2A will be a challenge, as that will be the point where there's maximum power dissipation in the transistor. Several transistors could be connected in cascode configuration to split the voltage across the devices.

This is the electronic load I now have and its maximum voltage rating is 360 VDC / 30 A / 300 watt. Trying to find some load that works at a higher voltage might be a problem at a reasonable cost. This load uses MOSFETs not BJTs.

This is the electronic load I now have and its maximum voltage rating is 360 VDC / 30 A / 300 watt. Trying to find some load that works at a higher voltage might be a problem at a reasonable cost. This load uses MOSFETs not BJTs.

Thank Youthe idea to go with resistors came to me becouse Mosfet SOA is small for linear modeCan You post schematic of power stage of Yours load?

HiI need for simple load to test supply 400V 0-0.4ACan I use common 30V transistor load and power resistor in series?

What you need is a classic case for a MOSFET design using LINEAR devices from IXYS. The normal switching types are not suitable because the can easily killed by thermal runaway in the linear region. Dont forget, you are talking about 160W of dissipation (I would use 4 transisors on a fan cooled heatsink).

The idea of using series resistors to reduce MOSFET voltage and power is possible, but unelegant aqnd cumbersome.

There are a lot of electronic load circuits, even here in this forum. Most are based on an Op-Amp regulator. Pick one, get the right linear MOSFETs, and it should work.